One field of application of the invention is in actuators having small motors that have a compact construction and a light weight, particularly having small-scale brushless DC motors which are used in the automotive industry, for example, as a small motor actuator, a fan motor, a drive for flap actuators used, for example, in air conditioning units and for cooling the motor etc., the invention not being limited to these applications.
The drive unit according to the invention is particularly suitable for those kinds of actuators that, compared to the rotational speed of the electric motor, generate a slow movement of the actuator, such as in a ventilation flap. Brushless DC motors may be used as the drive motor that can generate a rotational speed of the drive shaft of up to some 4000 rpm, for example, in the range of 500 to 2500 rpm, whereas the required rotational speed at the output side for the flap actuator is, for example, in the range of 3 to 10 rpm. The required torque is in the range, for example, of 1 to 2 Nm. In these kinds of actuators, the drive unit has to thus generate a transmission ratio of greater than 1 (gear reduction) between the drive and output side, where the transmission ratio lies, for example, in the range of 2 to 8 or of 4 to 5 per gear stage, i.e. 4≦i≦5 for i=zoutput/zdrive, where z indicates the number of teeth in the gear wheels of a gear stage. The transmitted torque acts conversely to the rotational speed. The lower the rotational speed on the output side in relation to the rotational speed on the drive side, the higher the increase in torque from the drive to the output side.
In practice, for these kinds of drive units that use multi-stage gearing, there arises the problem that the overall torque transmissible by the actuator—alongside the electric motor—is determined by the gear stage that is subject to the greatest load, i.e. the gear stage directly before the output side. In theory, the gear stages have to be designed in terms of strength in relation to the transmission ratio from one gear stage to the next, which means that the gear wheels of higher gear stages have to be made of a resilient material having thicker shafts and/or have measures for stabilizing the teeth. The transmission of an increasing torque from one gear stage to the next results in a greater load on the individual gear wheels, particularly the teeth, as well as in greater deflection of the shaft of the respective higher gear stages and of the output side. The gear unit has thus to be designed such that it can withstand the increasing load in the higher gear stages. One method, for example, is to make the shafts thicker although this requires more installation space. Another method is to make the gear wheels of the last gear stage and those on the output side not only more stable in themselves, but also in their connection to the shaft. For this purpose, they can be made bigger and/or out of more high-quality, stable plastics or possibly provided with metal bushings that are molded, for example, into the gear wheels as an insert in a metal injection-molding process. However, these solutions are complex and expensive, they increase space requirements and possibly lead to new problems caused by the combination of materials when using different plastics and/or metals.
From the prior art, transmissions having power splitting are basically known that have two counter shafts whose gear wheels mesh with a common first gear wheel as in DE 10 2008 038 069 A1. From DE 195 14 361 C2 a torque splitting device for the even distribution of torque to a first drive shaft and a second drive shaft is known, where two identical toothed gear wheels mesh with a gear wheel. The former document finds application in wind power plants where the slow rotational speed of the main rotor has to be transformed into a fast generator speed. The field of application for the second document is in the area of helicopters, so as to transmit the rotational movement that is generated by a motor to the main rotor of the helicopter. These documents certainly show that, for the transmission of power in a gear unit, it is basically known to distribute the torque to two gear wheels, however, these documents do not provide a suitable solution for an actuator based on small motors.
From DE 195 47 513 C2, an adjuster for vehicle seats is known having a pivoting operating lever that is fixedly connected to a toothed area that in its center position engages simultaneously with two axially parallel gear wheels. These gear wheels are connected through oppositely aligned freewheels to aligned gear wheels that mesh with a common output wheel. On moving the operating lever out of its center position, this toothed area only engages with one of the two parallel gear wheels, so as to increase the pivotal range of the lever. Again the solution described in this document is not suitable for an actuator based on an electric motor; it is limited to a manual lever adjustment.
It is an object of the invention to provide a drive unit for an actuator based on an electric motor that realizes a transmission ratio greater than one from the drive wheel of the electric motor to the output side over a plurality of gear stages, wherein the drive unit is to be optimized for the torque it has to transmit.